1. Field of the Invention
The present invention relates to a semiconductor device having a nitride semiconductor layer formed on a silicon substrate.
2. Description of the Related Art
Costs for high-voltage resistant nitride semiconductor elements are reduced by forming a nitride semiconductor layer on an inexpensive silicon substrate. However, there is a significant difference in the lattice constant between a silicon substrate and a nitride semiconductor layer. Also, a silicon substrate and a nitride semiconductor layer are different in coefficient of thermal expansion. Therefore, large strain energy is generated in a nitride semiconductor layer formed on a silicon substrate by epitaxial growth. As a result, cracks and deterioration of crystal quality could easily occur in the nitride semiconductor layer.
In order to avid such a problem, a buffer layer made of nitride semiconductor is arranged between a silicon substrate and a functional layer formed of nitride semiconductor. Also, a method for arranging an aluminum nitride layer between a silicon substrate and a buffer layer has been proposed.
However, it is difficult to form an aluminum nitride layer having good flatness directly on a silicon substrate. There have not been sufficient studies carried out on influences that flatness of an aluminum nitride layer formed on a silicon substrate has on reliability of a semiconductor device. Hence, there is a concern that reliability of a semiconductor could be decreased.